1. Field of the Invention
The present invention relates generally to mobile radiocommunication systems.
2. Description of the Prior Art
In mobile radiocommunication systems, various processing operations are necessary to format information to be transmitted in a form suitable for its transmission to the radio interface.
In particular, coding (such as error correcting coding in particular) intended to introduce redundancy into the information transmitted provides protection against transmission errors. The coding rate is defined as the ratio of the number of information bits to be transmitted to the number of bits transmitted or bits coded. The coding is generally effected on sequences of information bits or blocks. For given allocated radio resources, the higher the coding rate the higher the information bit rate. However, a high coding rate necessitates good radio conditions, and otherwise quality of service is degraded.
Additional protection against transmission errors is generally provided for data services, generally by retransmitting blocks received incorrectly using a technique known as ARQ (Automatic Repeat reQuest). The blocks received incorrectly can be blocks in which errors are detected by an error detecting code or can be corrected by an error correcting code. The receiver signals the correct or incorrect status of the blocks received to the transmitter by means of acknowledgement messages (ACK) or non-acknowledgement messages (NACK).
Another processing operation is modulation to obtain an analog signal conveying information to be transmitted. Various modulation techniques are known in the art, characterized by their spectral efficiency, i.e. their ability to transmit a larger or smaller number of bits per symbol for the same allocated frequency band. In the GPRS (General Packet Radio Service) system, for example, there is only one form of modulation, GMSK, which transmits one bit per symbol, and in the EGPRS (Enhanced General Packet Radio Service) system there are two types of modulation, GMSK, which transmits one bit per symbol, and 8PSK, which transmits three bits per symbol. The higher the spectral efficiency of the modulation, the higher the bit rate of bits transmitted. However, a high spectral efficiency necessitates good radio conditions and quality of service is otherwise degraded.
Various techniques can be used to optimize the performance of the above systems, such as the following techniques in particular:
link adaptation: this technique dynamically adapts the coding and/or modulation scheme used as a function of radio conditions; in particular, when radio conditions are good, the coding rate can be increased and/or a type of modulation with higher spectral efficiency can be used, to increase the bit rate; for example, as defined in the document “GSM 03.64 Version 8.2.0 Release 1999” published by the ETSI, there are four possible coding schemes (CS1 TO CS4) in the GPRS system and nine possible modulation and coding schemes (MCS1 to MCS9) in the EGPRS system;
cell reselection; because the above systems generally have a cellular architecture, this technique selects a best cell to which to hand over a call in progress in accordance with radio criteria and possibly in accordance with other criteria;
etc.
It is therefore very important in the above systems to be able to evaluate the radio conditions as accurately as possible; otherwise performance can be degraded.
The quality of a radio link is generally represented by one or more quality indicators, such as the raw bit error rate (BER), the block erasure rate (BLER), the signal-to-interference ratio (SIR), etc.
The raw BER is obtained by comparing data received before error correcting decoding with corresponding data obtained after error correcting decoding followed by recoding using the same error correcting code as the transmitter.
The BLER corresponds to the rate of data blocks received incorrectly. If the retransmission technique is used, unlike other quality indicators, the BLER can also be determined at the transmitter, from the ACK/NACK messages transmitted by the receiver. Algorithms such as link adaptation or cell reselection algorithms, for example, are generally used in the network, so that the network can itself determine the BLER for the downlink direction without it being necessary for the mobile station to report to the network the BLER value that it determines.
A drawback of quality indicators like the raw BER or the SIR is that, unlike the BLER, they require an estimate, whence a risk of errors and therefore of degrading system performance.
Another drawback of quality indicators such as the raw BER or the SIR is that they are not directly representative of performance. These quality indicators are in fact representative of the quality of the transmission channel, and the relation between the quality of service and the quality of the transmission channel is not fixed, but depends on many factors such as the environment, the speed of the mobile station, etc. From this point of view, the BLER criterion is more pertinent or more representative of performance.
However, the BLER is not the most pertinent criterion in all cases.
In systems in which several coding and/or modulation schemes are possible, for example, such as the GPRS and EGPRS systems, for instance, unlike other quality indicators such as the raw BER or the SIR, the BLER depends on the coding and/or modulation scheme used.
Accordingly, for the GPRS system, the document WO 99/12304 proposes to estimate the value of the BLER that would be obtained for each coding scheme from statistical calculations on quality indicators obtained, such as the raw BER or the SIR, combined with the results of preceding trial and error or simulations. The document therefore does not propose to use quality indicators other than the raw BER or the SIR, and proposes a complex technique for estimating the BLER from the quality indicators obtained in this way, such as the raw BER or the SIR.
Similarly, if the technique of retransmitting blocks received incorrectly is used, the BLER is not the most pertinent criterion either.
There are other cases in which the BLER does not constitute a satisfactory criterion, in particular if the incremental redundancy technique is used. This technique is a more sophisticated and more effective retransmission technique than that previously mentioned. Instead of retransmitting a block always with the same coding scheme, it retransmits the block with a different coding scheme each time, and uses all successive retransmissions of the block conjointly to reduce the probability of decoding it incorrectly. All the blocks can then be transmitted without protection on their first transmission. For example, the effective coding rate is 1 after the first transmission, ½ after the first retransmission, ⅓ after the second retransmission, and so on. This technique, like the link adaptation technique, can therefore increase the usable bit rate, but as the BLER can then be much higher than in the case where this technique is not used, this criterion is not the most pertinent one either.
The document GB 2 330 737 proposes a quality indicator obtained by determining the difference between the number of packets received correctly or the number of packets whose transmission is required and the number of packets actually transmitted. However, this is not the most pertinent criterion either, especially if techniques such as the link adaptation or incremental redundancy techniques previously referred to are used.
There is therefore a need for a quality indicator that would in particular circumvent the various drawbacks previously cited and that would in particular be pertinent for most systems, although accurate and relatively simple to calculate.